As the fishes live at the bottom of the ocean of water, mankind lives at
the bottom of an “ocean of air.” (See Fig. 2.) No one is absolutely
certain about the depth of this air, but it has been estimated as low as
forty miles and as high as two hundred miles. Balloons have gone up to a
height of nearly nineteen miles (100,320 feet). We do know that the
higher we go, the thinner the air becomes. It is practically impossible
for man to ascend into the air more than five or six miles, owing to the
fact that the air above that height is so thin that there is not enough
to breathe. Naturally, the air at the bottom is more compact because of
the vast amount of air above. The air is a great weight lying upon
us—14.7 pounds per square inch of surface.